1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) system using a spread spectrum communication scheme and, more particularly, to an apparatus applicable to a CDMA system for controlling the transmission power of a mobile communication terminal.
2. Description of the Background Art
Currently, CDMA systems are attracting increasing attention as a multiple access system which allows a plurality of users to hold communication at the same time while sharing a radio transmission path. The CDMA system assigns the same frequency to a plurality of users by use of a spread spectrum technology. A study on spread spectrum technology is discussed in, e.g., Dohi and Sawahashi xe2x80x9cPower Control by employing Interference Power for DS/CDMAxe2x80x9d, Technical Report of IEICE, the Institute of Electronics, Information And Communication Engineers of Japan, RCS 94-99 (1994-10), pp. 63-68. Accurate control over transmission power is essential with the CDMA system in increasing the subscriber capacity. A conventional transmission power control procedure is as follows.
First, a base station observes a signal-to-interference power ratio (SIR hereinafter) and compares it with a required SIR selected beforehand. If the observed or actual SIR is greater than the required SIR, meaning that communication quality is high, the base station inserts a command for lowering the transmission power in transmission power control bits included in a frame to be sent to a mobile station. If the actual SIR is smaller than the required SIR, meaning that communication quality is low, the base station inserts a command for raising the transmission power in the above control bits.
It is a common practice with a transmission power control apparatus to insert the transmission power control bits in a data signal, so that the base station can control the transmission power of the individual mobile station. Mobile stations each demodulates the frame received from the base station and raises or lowers its transmission power as instructed by the control bits included in the frame.
However, the problem with the conventional transmission power control apparatus is that it controls the transmission power of the individual mobile station on a call-by-call basis, neglecting the degrees of importance of information media. As a result, when the number of calls is great, the power of interference waves increases and deteriorates even the SIR of information medium needing high quality (usually data communication medium). In this condition, the control apparatus sends a transmission power increase command for calls meant for the information medium needing high quality. This, however, deteriorates the SIR when it comes to calls meant for the information media whose required quality is relatively low. Consequently, the control apparatus commands even the information media not needing high quality to raise their transmission powers. In the worst case, the control apparatus repeats the above procedure and eventually deteriorates the SIRs of all the information media, failing to maintain communication quality.
It is therefore an object of the present invention to provide a transmission power control apparatus for a CDMA system capable of reducing the deviation of the actual quality from required quality more with an information medium of higher degree of priority than with an information medium of lower degree of priority,
In accordance with the present invention, a transmission power control apparatus determines, with each of signals received from a plurality of terminals, a difference between a power ratio to a total power and a requested value, and control a transmission power to be requested of an associated one of the plurality of terminals on the basis of the difference. A request signal power updating circuit weights the difference determined signal by signal in accordance with signal-by-signal required communication quality to thereby calculate an evaluation value representative of the evaluation of the difference, and updates the transmission power to be requested of the terminal associated with the above difference such that the evaluation value decreases.
Also, in accordance with the present invention, a transmission power control apparatus controls the transmission power of each of a plurality of terminals on the basis of the power of a signal received from the individual terminal. A power ratio calculating circuit determines, with each of a plurality of information media available for the plurality of terminals, a sum of powers received from the terminals, and determines a ratio of the sum of powers to a total power received from all of the plurality of terminals. A difference calculating circuit calculates a difference between the ratio determined medium by medium and a request value requested a single control interval before terminal by terminal. A request signal power updating circuit weights the difference in accordance with the degree of importance of the information medium to thereby produce an evaluation amount of the difference for each information medium, and updates a request signal power to be requested of the individual information medium such that the evaluation value decreases.
Further, in accordance with the present invention, a transmission power control apparatus controls the transmission power of each of a plurality of terminals on the basis of the power of a signal received from the individual terminal. A power ratio calculating circuit determines, with each of a plurality of information media, a sum of powers of signals received from the terminals, and determines a ratio of the sum of powers to a total power received from all of the plurality of terminals. A correction amount calculating circuit produces a difference between the ratio determined medium by medium and a request signal power ratio produced from a request signal power calculated a single control interval before medium by medium, weights the difference in accordance with the degree of importance of the information medium to thereby evaluate the difference, and calculates a correction amount medium by medium such that a resulting evaluation value decreases. A sign identifying circuit determines whether or not correction amounts output from the correction amount calculating circuit each is positive or negative. A step control circuit outputs, for each of the results of identification output from the sign identifying circuit, a preselected increment/decrement in accordance with the result. A request signal power calculating circuit adds the preselected increment/decrement to the request signal power calculated a single control interval before medium by medium to thereby output a request signal power to be requested at the current control interval. A control command transmitting circuit transmits, based on the request signal power output from the request signal power calculating circuit, a control signal to a corresponding one of the plurality of terminals.